Mobile and portable radio frequency transceivers and other communications devices have heretofore incorporated noise blanker circuitry which detect impulse noise signals that have sufficient peak power to interfere with an FM receiver listening to weak signals. The function of such noise blankers is to eliminate ignition or other vehicle-generated noise which might interfere with radio reception. Such impulse noise may emanate from the vehicle tires, the alternator, the ignition system, etc.
A noise blanker also must be designed to eliminate noise signals received from other sources of interference external to a radio installed in a motor vehicle. For example, interfering noise signals may be generated by electrical arc-welding equipment or any other source which radiates intense, narrow signals in the RF region.
Impulse noise from any of such sources is typified by high peak power, but very low average power. This is because the energy occurs in very short bursts, 10-20 nanoseconds in extent, spaced relatively far apart.
Interference from continuous-wave (CW) signal sources, such as pagers or broadcast television, can serve to block the proper operation of noise-blanker circuitry. In this regard, noise blankers are typically designed to detect and cancel impulse noise signals which would otherwise interfere with the reception of weak FM signals. For modern lowband (30 to 50 MHz) land-mobile receivers, this means that the detection threshold of the noise blanker should be at a low level. With the detection limit set low, the noise blanker is subject to capture by continuous- wave signals or other high-duty-cycle signals which occur in the sampling band, when their signal strength is equal to or greater than the noise detection threshold.
As noted above, such continuous-wave signals can interfere with the proper functioning of the noise blanker circuitry. The strength of these CW signals is not enough to trigger the operation of the noise-eliminating circuitry within the noise blanker. However, the CW signal interferes with the proper gating in the noise blanker which ultimately results in noise heard by a radio listener.
The noise-blanker circuitry in performing its function of eliminating impulse noise must respond to a broad frequency band. Accordingly, even if the continuous-wave signal is outside the bandwidth of the receiver, it nevertheless may be sufficiently strong to adversely affect the noise-blanker circuitry.
The noise blanker method and apparatus of the present invention prevents continuous-wave or continuous-wave-like signals from significantly interfering with noise-blanking operations. Such continuous-wave interfering signals heretofore prevented prior art noise blankers from optimally blanking out noise from received signals and, in some instances, prevented noise from being eliminated at all.
The noise blanker of the present invention exhibits improved performance over prior art noise blankers in this regard, by utilizing a gain control circuit which responds only to the average power in a noise blanking channel to control the channel gain so that CW-type signals are always held below the blanker threshold. In this fashion, the noise blanker is provided with a significant degree of immunity to capture by continuous-wave signals.
Noise blankers in the prior art have typically been disposed at the output of the transceiver's RF preselection circuitry. Advantageously, the noise blanker circuitry of the present invention is disposed to operate on the transceiver's intermediate frequency input signal. The intermediate frequency input signal is a converted frequency signal which is produced by mixing the RF input signal with the signal produced by a local oscillator in a mixer stage to obtain a signal having a frequency equal to the difference between the mixer input signals. In this fashion, the present noise blanker circuit operates on a sample of noise which is as close as possible to the receiver bandpass.